The Latin language is deeply embedded in English. This is doubly so in the case of the ampersand, “&”: both the name and the shape of the letter derive from Latin.
When spelling aloud, English students in the 17th and 18th centuries would add the phrase “per se” before any letter that constituted a word “by itself” (e.g. “I”, “a”, “O!”). The ampersand was also recited at the end of the alphabet, thus: “…X, Y, Z, and per se And”. When slurred and elided, this be came “ampersand”. (Phonologically, ndp> np > mp).
The shape of the ampersand derives from the Latin word et, meaning “and”. Because of its high frequency in texts, it was written with a ligature (and joining of the two letters) in Old Latin cursive. By the 9th century, the et took this form in the Book of Kells: